As shown in FIGS. 1 and 2, a bicycle crankset of the prior art comprises a chain ring 1, a sleeve 2, two bearing sets 3, two bearings 4, two fastening nuts 5, two rubber washers 6, a retaining washer 7, a tightening nut 8, and a crankarm 9. The crankset is fastened on the crank arm 9 by the tightening nut 8 which is engaged with the outer threads 10 of the crank arm 9. The two rubber washers 6 have a square cross section and are intended to be lodged in the circular groove 11 of the fastening nut 5 so as to bring about the dustproof and the waterproof effects. The fastening nuts 5 are provided with a lubricating grease, which prevents the rubber washers 6 from being lodged in the circular grooves 11 of the fastening nuts 5 with precision. The quality of the prior art bicycle crankset is thus seriously undermined. In addition, the assembly of the bicycle crankset of the prior art can not be done efficiently.